There are many applications which require the recording of hundreds or thousands of images per second. Such applications include automobile crash testing, missile range instrumentation and fault detection in high speed machinery. Typical high frame rate video cameras employ a multi-channel sensor to minimize channel bandwidth and data rate. (See, e.g., U.S. Pat. No. 4,322,752, issued Mar. 30, 1982, inventor Bixby). One of the difficulties to be dealt with in using such a sensor is the requirement that the transfer characteristic, relating output signal amplitude to sensor illumination, be closely matched across all channels. Under uniform sensor illumination, matching errors larger than approximately 1% can be readily detected by the human eye. If the transfer function is linear, uniformity can be achieved by adjusting individual channel gain and offset.
In a known color fast frame imaging system, a sixteen channel sensor is employed, wherein the sensor is read out as sixteen parallel channels of analog video signals. The system electronics supports the sensor with sixteen independent channels of analog and digital processing including an analog-to-digital converter (ADC). Two controls control the respective channels gain and offset by adjusting the top and bottom ladder electrical potentials of the respective channel's ADC. In the known procedure, each channel's offset control was adjusted while the sensor was capped (kept in darkness) to balance each channel's average video level to the system black setup value. Subsequently, each channel's gain control was adjusted to balance each channel's average video value to the global average video level in the presence of a broad-band flat field light source.
A drawback to this procedure is that it assumes a linear response, but indirect evidence has shown that a non-linear response may be occurring. When left uncompensated for, this lack of linearity in the sensor response has implications beyond a non-linear rendering of intensity values. The greatest impact of the non-linearity is in regards to its effects on the color reproduction. Any departure from the ideal response immediately begins to color the rendering of neutrals in the scene for typical lighting situations. This is because such departures can be looked at as an offset from the ideal response. This offset is multiplied by the white balance gains of which there are three different gains for each of the three color components. Thus, this singular offset now behaves as three different offsets for each of the color planes. When these offsets are added to the rendering of a neutral scale, the result is a false coloration of the scene. Furthermore, the very nature of the non-linearity has been seen to vary from channel to channel. Thus, the false coloration of neutrals can appear to vary abruptly across channel boundaries.